Key-ways is not recommended if it requires to alternate the directional loads and shocks.
Key-ways create shaft imbalance.
The shaft strength tends to reduced due to the extra depth of the key-ways.
The assembling and disassembling of the coupling onto the shaft gets lengthy and strenuous. There are high chances that the shaft may get damaged during the dismantling process adding cost.
Expensive – As the key-way needs to be machined on both the shaft and the coupling.
Key-ways reduce the capacity to transmit the power.
There is no axial movement between the shaft and the hub.
The keyed joints are over dimensioned – since it is difficult to calculate accurately.